ARNOLD — THE PALEONTOLOGY AND STRATIOIJAPH V OK SAN PEDRO. 343 



One valve \va^^ fouiul in tlie lower San Pedro scries of Deadman Islaiid, and 

 hvii in the Pliocene of the same locality. One is i)rol)al)ly an end valve and is 

 distingnishai)le l)y its strong convexity, thickness, and smooth surface. It has a 

 small, acute notch on each side near the end. 



Dimensions.- — Long. 20 mm.; lat. 30 mm.; alt. \-'> mm. 



Lirinr/. — Straits of Fiioa to Monterey; Kamtst-liatka (Cooper): Hakodate, 

 .lapan (?) (Carpenter). 



P leistocene.-— San Diego (Cooper): San Pedro (Arnold). 



Pliocene. — Deadman Island, San Pc(ho (Arnold). 



Supcrfamily Ol'SlL'ii I'l"( )X 1 .\. 

 Family LXXXIX. MOrALIlD^E. 



Genus Mopalia Gray. 



Shell regular; lamina' lengthened; anterior valve with si.x or more slits, the others with a 

 single slit; last valve sinulate behind; sinus narrow, mucro median, depressed; sutures indented; 

 girdle wide, bristly, sometimes fissured behind, sometimes projecting anteriorly. 



Mopalia hlainvillei Brod. is a characteristic species. 



405. Mopalia ciliata Smverby. 



Chilon muscosus Gld., Proc. Bost. Soc. Nat. Hist., Vol. II, 1846, p. 145; Wilkes' Expl. Exped., 



Vol. XII, p. 313, fig. 436, 1852. 

 Mopalia nmscosa Gld., Cpr., Brit. Assn. Rept., 1863, p. 64S. 

 Mopalia ciliata Siiv., Keep, West Coast Shells, p. no, fig. 98, 1892. Williamson, Proc. U. S. 



NaL Mus., Vol. XV, 1892, p. 196. 



Shell depressed, rather broad, oval; obliquely ridged along the back; sculpture prominent 

 and covering whole of surface; side areas sculptured with granulated, radiating ridges; central area 

 ornamented with longitudinal, granulated, rai.sed lines; mucro median depressed, ornamented with 

 sharp, oblique lines which meet on middle and form an acute angle open anteriorly; anterior valve 

 large, semicircular, with ten granulated, elevated, radiating lines, and interspaces granulated as in 

 lateral areas. 



Dimensions. — Long. 40 mm. ; lat. 19 mm. ; depth 6 mm. 



Description from living specimen. The only specimen of this species ever 

 reported in the fossil state was found in the upper San Pedro series of San Pedro. 

 It is a perfect anterior valve, 10 millimeters wide and 5.2 millimeters long. 



Living. — Vancouver to Monterey (Carpenter): San Pedro (Williamson). 



Pleistocene. — San Pedro (Arnold). 



